Shortly after the general election, the new government will conduct a spending review which even on Labour’s numbers will result in further cuts for non-protected departments – MoD being the largest.

Unless things change, as RUSI’s Malcolm Chalmers showed last autumn, the best the ministry can hope for is “flat cash”, with defence spending falling to 1.5 per cent of GDP, £10.8bn (23.2 per cent) lower than the 2 per cent NATO target for 2020/21.

This fiscal backdrop inevitably focuses attention on the MoD’s priorities, and in particular its equipment programme. From 2018 to 2032, between a quarter and a third of the equipment budget – each year, ever year – is slated to be spent on the replacement Trident submarines.

Any cost overruns – which are likely as the other variables of schedule or capability can’t be varied – will come at the expense of the conventional forces, and the UK’s ability to operate globally.

As Nick Harvey MP made clear in the Commons Trident debate Trident on 20 January, this could well mean cutting the Army to 60,000 men – a previously unthinkable figure that would render us effectively impotent internationally.

It need not be so. Today, CentreForum has published a paper outlining how a system less capable – and much less costly – than Trident can provide the UK with a credible, minimum, independent nuclear deterrent.

It draws on the recently declassified government definition of minimum deterrence developed to deter the Soviet Union during the depths of the Cold War in 1982. This minimum deterrent – called Duff-Mason, after the two study leaders – was defined as the ability to destroy 10 Soviet cities other than Moscow or Leningrad, or to deliver 30 warheads against Soviet targets.

Today, Russia has less coercive power over its population than the Soviet Union of Brezhnev and Andropov, likely reducing this minimum deterrence threshold further. However, to demonstrate the conservative assumptions our paper makes, we’ve used the Cold War definition.

Our proposal uses a British-built version of the new US precision-guided B61-12 thermonuclear bomb delivered through the UK’s forthcoming F-35 Joint Strike Fighters, operating from land bases and from the Royal Navy’s new carriers.

The advantage of this proposal is that by using dual-role systems, the nuclear mission could free-ride on much of the capital and operating costs of the conventional forces. It would significantly reduce costs.

To provide the most effective support, the RAF’s 14 existing Voyager tankers would be converted to receive and off-load fuel with the high-capacity “flying boom”, allowing the RAF’s new RC-135 Rivet Joint electronic intelligence aircraft to support operations worldwide, as well as the UK C-17 fleet for the first time.

All of these forces to support the nuclear mission would provide a significant enhancement to the UK’s conventional global force projection capability. Better, over and above this, our proposal’s savings provide an additional £5–13bn for the conventional force’s recapitalisation out to 2032.

By retaining the submarine industrial base, the facilities and expertise at Aldermaston, and the UK’s uranium and plutonium stocks, in the very unlikely case that there is a new Cold War, we can return to Trident. After all, the UK went from a standing start to Polaris patrols in six years in the 1960s.

The result would be a much more capable conventional force, which balances the conventional mission and the UK’s global role with a credible, minimum independent UK nuclear force fit for the 21st century.

The alternative diminution of the UK’s international role by clinging on to a single-role Cold War weapon, and slashing the conventional forces to pay for it, is far too high a cost to pay.